Support for freedesktop.org sound themes in QSoundEffect

The freedesktop sound theme (https://specifications.freedesktop.org/sound-theme-spec/sound-theme-spec-latest.html) works very similar to an icon theme.

Instead of specifying a specific audio file that should be played a semantic sound name (e.g. "dialog-warning") is specified which is then resolved from a user-configured sound theme.

My proposal is to add the ability to specify a sound by name instead of URL to QSoundEffect that is then resolved via the sound theme.

Both the name->file resolution as well as the actual playback can be done using the reference implementation library, libcanberra (http://0pointer.de/lennart/projects/libcanberra/). It also allows to play regular audio files by URL. It is optimized for low-latency effect playback so it fits well within the scope of QSoundEffect.